I've done this before in my electronics classes. The second DC motor makes a wonderful generator and load. If you short the driven motor's leads, you can often bring the driving motor to a dead stop, enough that if you leave it that way, you'll burn it up. I used DECENT DC motors, not toy car motors. I like the 12-volt variety with carbon brushes, the more commutator segments, the better. Adding a rheostat to the "output" of the driven motor allows you to vary the load. Using a lamp bank, switching in parallel lamps is good too, as it gives a visual indication if you vary the speed of the driving motor.